Papers by Author: Nicolò Piluso

Abstract: In this paper we investigate the role of the growth rate (varied by changing the Si/H₂ ratio and using TCS to avoid Si droplet formation) on the surface roughness (R_q), the density of single Shockley stacking faults (SSSF) and 3C-inclusions (i.e. epi-stacking faults, ESF). We find that optimized processes with higher growth rates allow to improve the films in all the considered aspects. This result, together with the reduced cost of growth processes, indicates that high growth rates should always be used to improve the overall quality of 4H-SiC homoepitaxial growths. Furthermore we analyze the connection between surface morphology and density of traps (D_it) at the SiO₂/SiC interface in fabricated MOS devices finding consistent indications that higher surface roughness (step-bunched surfaces) can improve the quality of the interface by reducing the D_it value.

Abstract: Two dimensional maps of the electronic conductance in epitaxial graphene (EG) grown on SiC were obtained by conductive atomic force microscopy (CAFM). The correlation between morphological and electrical maps revealed the local conductance degradation in EG over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at 1L/2L junction is a purely quantum mechanical effect, due to the weak coupling between 1L and 2L electron wavefunctions.

Abstract: In this article we compare the strain distribution observed in 3C-SiC/Si (100) cantilevers, using the shift of the transverse optical (TO) mode in micro-Raman maps, with the values predicted using a recent analytic theory []. By taking advantage of an under etching of the microstructures during the fabrication processes, that removes a thin layer of highly defective SiC close to the film/substrate interface near the edges of the microstructures, we show that the variation of the experimental measured strain can be ascribed to a non-linearity of the strain field along the 3C-SiC film thickness.

Abstract: Micro-Raman spectroscopy has been used to study the dependence between the carrier concentration and electrical mobility in n-doped 3C-SiC films grown on (111) and (100) Si oriented substrates. Local stacking variations observed on the (111) 3C-SiC surface lead to a worse crystal morphology compared to (100) 3C-SiC films resulting in a decrease of the average bulk mobility.

Abstract: In this article, using finite element simulations and analytical approaches, we demonstrate that planar rotators[1] can be effectively used to determine both the uniform and gradient residual stresses in thin films with higher accuracy compared to other microstructures.

Abstract: Raman microscopy has been used to study the stress distribution on 3C-SiC/Si(100) micro-machined free standing structures. Linear scans along different structures reveal similar trends of the TO mode Raman Shift. We have found that, independently of the microstructure considered, the Raman frequency decreases close to the undercut. We compare our experimental measurements with FEM simulations finding that, close to the undercut, the stress tensor becomes non-diagonal, modifying the Raman shift to stress relation.

Abstract: SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on either bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication, but the high residual stress created during the film grow limits the development of the material for these applications. In order to evaluate the amount of residual stress released from the epi-film, different micro-machined structures were developed. Finite elements simulations of the micro-machined structures have also been carried out in order to evaluate, in detail, the stress field inside the structures and to test the analytical model used. With finite element modeling a exponential approximation of the stress relationship was studied, yielding a better fit with the experimental data. This study shows that this new approximation of the total residual stress function reduces the disagreement between experimental and simulated data.

Abstract: The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on either bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication. In this work a comparison between single crystal and poly crystal 3C-SiC micro-machined structures will be presented. The free-standing structures realized (cantilevers and membrane) are also a suitable method for residual field stress investigation in 3C-SiC films. Measurement of the Raman shift indicates that the mono and poly-crystal 3C-SiC structures release the stress in different ways. Finite element analysis was performed to determine the stress field inside the films and provided a good fit to the experimental data. A comprehensive experimental and theoretical study of 3C-SiC MEMS structures has been performed and is presented.

Abstract: Shockley-type Stacking faults (SSF) in hexagonal Silicon Carbide polytypes have received considerable attention in recent years since it has been found that these defects are responsible for the degradation of forward I-V characteristics in p-i-n diodes. In order to extend the knowledge on these kind of defects and theoretically support experimental findings (specifically, photoluminescence spectral analysis), we have determined the Kohn-Sham electronic band structures, along the closed path Γ-M-K-Γ, using density functional theory. We have also determined the energies of the SSFs with respect to the perfect crystal finding that the (35) and (44) SSFs have unexpectedly low formation energies, for this reason we could expect these two defects to be easily generated/expanded either during the growth or post-growth process steps.

Abstract: Raman microscopy has been used to study transport properties in hetero-epitaxial 3C-SiC/Si thin films. By an accurate analysis of the Longitudinal Optic phonon-plasmon coupled (LOPC) modes in n-type doped 3C-SiC films, free carrier density and mobility has been determined. A study of doped 3C-SiC reveals a strong relationship between the calculated free carrier density and both the C/Si ratio used during the epitaxial process and Silicon substrates orientation on which 3C-SiC thin films were grown (maintaining the N2 gas flow rate). The free carrier density obtained is in the range between 5x1016 cm-3 and 4x1018 cm-3. Epitaxial films grown on (111) Si substrates show a higher free carrier density and a lower dependence on C/Si ratios as compared to films grown on (100) Si substrates.